This invention relates to an adaptive receiver for use in reducing waveform distortion caused by fading occurred in multipath channels.
Various types of adaptive receiver have been studied for a variety of fading multipath channels. As typical examples of the adaptive receiver, first and second types are well known in the art. The first type is that utilizes an MMSE (minimum mean square error) method while the second type is that utilizes a correlation method.
The adaptive receiver of the first type comprises a primary transversal filter which is called a matched filter, a decision-feedback equalizer, and a secondary transversal filter which is called a replica filter. The matched filter filters an input signal received through a transmission channel in accordance with a plurality of tap coefficients and delivers a primary filtered signal to the decision-feedback equalizer. The decision-feedback equalizer carries out equalization operation and produces an equalized signal as a decision signal. The decision signal is supplied to the replica filter. The replica filter is for estimating an impulse response of the transmission channel by the use of an error signal and the decision signal and produces a replica signal representative of replica of the input signal.
For this purpose, the input signal is supplied to a difference calculator through an input delay circuit as a delayed input signal. The difference calculator calculates a difference between the delayed input signal and the replica signal and delivers a difference signal representative of the difference to the replica filter as the error signal. In the replica filter, adaptation is carried out by minimizing a mean square value of the difference, namely, an error. As a result of execution of the MMSE method, the replica filter further delivers a plurality of tap coefficient signals representative of the plurality of tap coefficients to the matched filter.
The adaptive receiver of the second type comprises the matched filter and the decision-feedback equalizer which are similar to those described above. The second type adaptive receiver further comprises another type of secondary transversal filter in place of the replica filter. The other type secondary transversal filter comprises a correlation unit. The other type secondary transversal filter is for obtaining complex conjugate of a time reversal impulse response of the transmission channel and carries out correlation operation by the use of the input signal and the decision signal. As a result of the correlation operation, the other type secondary transversal filter delivers a plurality of tap coefficient signals representative of the plurality of tap coefficients to the matched filter.
As apparent from the above description, although the first type adaptive receiver has construction different from that of the second type adaptive receiver, the first type is equivalent to the second type in principle. The first and the second type have the same characteristics, such as a tap coefficient divergence characteristic. This point is disclosed in an article contributed by Kaveh Pahlavan and James W. Matthews to IEEE Transactions on Communications, Vol. 38, No. 12 (December 1990), pages 2106 to 2113, under the title of "Performance of Adaptive Matched Filter Receivers Over Fading Multipath Channels".
As will later become clear, both the first and the second types of the adaptive receiver are not suitable for low symbol rate transmission. This is because matched filtering function by the matched filter and carrier recovery function become difficult to exist together as a transmission symbol rate becomes low.